Chapter 2 ferrous material structure and binary alloy system

CHAPTER 2 :FERROUS MATERIAL STRUCTURE AND BINARY ALLOY SYSTEM

IRON PRODUCTION

METAL PRODUCTION

INTRODUCTION TO IRON

Pure iron is soft (softer than aluminium), but is unobtainable by smelting.

Iron is significantly hardened and strengthened by impurities from the smelting process, such as carbon.

Crude iron metal is produced in blast furnaces, where ore is reduced by coke to cast iron.

Types of Iron Ore

LimoniteIron

Carbonate

Magnetite Hematite

IRON ORE CONTENT

LIMONITEContent : 20% - 55% irons + 40% waterColour : Yellow brownish

IRON CARBONATEContent : Less than 30% irons + Carbon + PhosphorousColour : Grey

MAGNETITEContent : 72.4% ironsColour : Black

HEMATITEContent : 40% - 65% ironsColour : Dark brown reddish

BLAST FURNACE

What is the purpose of Blast Furnace?Is to chemically reduce and convert iron ores into liquid iron called "hot metal". This is due to Iron ore contains impurities, mainly silica (silicon dioxide).

Basic material for iron production :Iron ore, limestone, and coke.

Blast Furnace diagram :

The Exhaust Gas OutletTo expel used gases

Charging BellsHave two bells; small and big. To allow the Charge drop into the Furnace.

Gas OutletHoles in the Furnace that allows the escaping gases to get to the Exhaust Gas Outlet.

TuyereThese are small pipes that permit hot air to enter the furnace.

TapholeUsed to draw off the molten Iron.

Slag HoleUsed to draw off the waste Slag.

Refractory LiningReflects the heat into the Furnace.

Conveyor SystemTakes the Charge to the top of the Blast Furnace. The Charge is carried in Skip Cars which run on a rail track.

Process in blast furnace

- Iron ore, coke and limestone are carried to the top of the blast furnace and dumped into it.

- Limestone is added to the blast furnace to remove the impurities in the iron ore.

- Limestone reacts with the silica to form molten slag in the blast furnace.

- Slag flows to the bottom of the furnace where it floats on the liquid iron and is easily removed.

- Hot air is blasted into the furnace causing coke to burn rapidly and raise the temperature to 2000°C.carbon+ oxygen = carbon dioxide + heat.

- The carbon dioxide then reacts with hot carbon to form carbon monoxide which reduces iron in the ore to iron metal.

- Iron falls to the bottom of the furnace and is tapped off periodically.

STEEL PRODUCTION

INTRODUCTION TO STEEL Is an alloy consisting of a certain

proportion of carbon (between 0.2% and 2.1%) and iron.

Steel is 1000 times harder than pure iron.

Further refinement with oxygen reduces the carbon content in cast iron from blast furnace process produce steel.

BASIC OXYGEN FURNACE (BOP) What is the process of Basic Oxygen

Furnace (BOF)?Is a steel making furnace, in which molten pig iron and steel scrap convert into steel due to oxidizing action of oxygen blown into the melt under a basic slag.

BOF Diagram

The Water-Cooled Oxygen LanceProvides the oxygen to the furnace so that the temperature in the furnace will increase.

The Slagging HoleWhere the slag can be poured out when necessary.

The Steel Shell

The Refractory Lining

Has two purposes. The first is to keep the heat from the furnace. The second reason is to protect the Steel Shell of the Furnace.

The Tapping HoleUsed to remove the Molten Steel from the Furnace.

The Gas Offtake HoodHas two purposes. i) To trap the dangerous gases that the BOF produces so that they cannot escape into the atmosphere. One important use of the gases is to heat the Oxygen that is going through the Water-Cooled Oxygen Lance. ii) To reduce the amount of heat loss in the Furnace.

The furnace is tilt and charge it with scrap.

Process in BOF

Hot iron metal from the blast furnace is poured from a ladle into the top of the tilted furnace.

The charged furnace is returned to an upright position and a water cooled oxygen lance is lowered from the top; oxygen is blown at supersonic speeds causing rapid mixing and heat from the oxidation of iron and impurities. Fluxes are added to help carry off the impurities in the floating slag layer.

After the steel has been refined, the furnace is tilted (opposite to the charging side) and molten steel is poured out into a preheated ladle.

ELECTRIC ARC FURNACE

What is the process of Electric Arc Furnace??Is a steel making furnace, in which steel scrap is heated and melted by heat of electric arcs striking between the furnace electrodes and the metal bath.

Electric Arc Furnace Diagram

Scrap Charge- Steel scrap is tipped into the EAF. - Electrodes then are lowered into the furnace. - An electric current is passed through the electrodes to form an arc. - The heat generated by this arc melts the scrap.

Melting Phase- During the melting process, other metals (sulphur) are added to the steel to give it the required chemical composition. - Oxygen is blown in to the furnace to purify the steel.- Limestone and fluorspar are added to combine with the impurities and form slag.

Tap out- The furnace is tilted to allow the slag, which is floating on the surface of the molten steel, to be poured off. - The furnace is then tilted in the other direction and the molten steel poured (tapped) into a ladle.

PLAIN CARBON STEEL

INTRODUCTION

Plain carbon steel is a type of steel having a carbon content may range from less than 0.02% to slightly more than 2%.

IRON-CARBON PHASE DIAGRAM

The eutectoid reaction describes the phase transformation of one solid into two different solids.

In the Fe-C system, there is a eutectoid point at approximately 0.8wt% C, 723°C.

The phase just above the eutectoid temperature for plain carbon steels is known as austenite or gamma.

The compositions of the two new phases are given by the ends of the tie-line through the eutectoid point.

The general eutectoid reaction is therefore:Solid γ –> solid α + solid β

Or using the names given to these phases:Austenite –> ferrite + cementite (Fe3C)

ASSIGNMENTi) Sketch an iron-carbon phase diagram up to 2% C & at 910ºC together with the microstructure for various phases of steel.ii) Create your own style how to remember every phase in the iron-carbon phase diagram.

Submit the assignment by 2nd October 2011.

Ferrite (α)- Also known as alpha iron.- Is an interstitial solid solution of a small amount of carbon dissolved in iron with a Body Centered Cubic (B.C.C.) crystal structure.

Austenite (γ)- Also known as gamma-iron, - Is an interstitial solid solution of carbon dissolved in iron with a face centered cubic crystal (F.C.C) structure.

Cementite (Fe3C)

- Is also known as iron carbide which has a chemical formula, Fe3C.

- It contains 6.67 % Carbon by weight.- Its crystal structure is orthorhombic.

Pearlite - It is the eutectoid mixture containing 0.83 % Carbon and is formed at 1333oF on very slow cooling. - It is very fine platelike or lamellar mixture of ferrite and cementite. - The structure of pearlite includes a white matrix (ferritic background) which includes thin plates of cementite.

Ledeburite (a + Fe3C)

- It is the eutectic mixture of austenite and cementite.- It contains 4.3 % Carbon and represents the eutectic of cast iron. - Ledeburite exists when the carbon content is greater than 2 %, which represents the dividing line on the equilibrium diagram between steel and cast iron.

ALLOY STEEL

DEFINITION

Alloy steel is a steel which contains more than 1% of other elements besides carbon and iron.

PURPOSE

To improve quality and steel properties so that it can easily be modify by heat treatment.

ELEMENTS INFLUENCE

Manganese (Mn)

improves hardenability, ductility and wear resistance. Mn, increasing strength at high temperatures.

Copper (Cu) improves corrosion resistance.

Chromium (Cr)

improves hardenability, strength and wear resistance, sharply increases corrosion resistance at high concentrations (> 12%).

Sulfur improves machinability.

Silicon (Si) improves strength, elasticity, acid resistance and promotes large grain sizes, which cause increasing magnetic permeability.

Nickel (Ni) increases strength, impact strength and toughness, impart corrosion resistance in combination with other elements.

Molybdenum (Mo)

increases hardenability and strength particularly at high temperatures.

Aluminum (Al)

deoxidizer, limits austenite grains growth.

Vanadium (V)

increases strength, hardness, creep resistance and impact resistance due to formation of hard vanadium carbides, limits grain size.

Tungsten (W)

increases hardness particularly at elevated temperatures due to stable carbides, refines grain size

Titanium (Ti)

improves strength and corrosion resistance, limits austenite grain size.

TYPES OF ALLOY STEELS

3 TYPES

LOW ALLOY STEELS

MEDIUM ALLOY STEELS

HIGH ALLOY STEELS

STRUCTURE STEEL CORROSION RESISTANCE STEEL HEAT RESISTANCE STEEL TOOL & MOULD STEEL MAGNETIC STEEL

Structure Steel

o Element Content : Ni, Mn, Cr, Moo Properties : High strengtho Usage : for construction purpose.

Corrosion Resistance Steel

o Element Content : Cr, Ni, Mo, Tio Properties : corrosion resistance, high

strength and ductility.o Usage : cutlery, health care and surgical

equipment, etc.

Heat Resistance Steel

Element Content : 18% W + 4% Cr + 1% V + 0.88% C

Properties : High strength and hardness, wear resistance.

Usage : Tool for cutting at high temperature ~660˚C.

Tool & Mould Steel

o Element Content : 0.6-1.5% Co Properties : High strength, wear

resistance at elevated temperature.o Usage : used in forming and machining

of metals.

Magnetic Steel

o Element Content : Depend on types of magnetic steel to produce (permanent or temporary).

o Properties : have magnetic field.o Usage : Magnet

CAST IRON

HOW CAST IRON BEEN MADE ???Re-melting pig iron, steel scrap to cupola (small blast furnace).

FACTORS INFLUENCE PROPERTIES

4 FACTORS

COOLING RATE

ELEMENT CONTENT

HEAT TREATMENT

CARBON CONTENT

Cooling Rate

Depend on thickness and type of mould.

High Cooling Rate

Low Cooling Rate

Produce : Cementite

Produce : Graphite

White Cast iron Gray Cast Iron

Element Content

Element Function

Carbon Increase graphite content

Silicon Help formation of graphite

Sulfur + Manganese

Stabilize cementite and combination of sulfur and manganese form manganese sulfide.

Phosphorous Lower down melting point

Heat Treatment

TYPES OF CAST IRON

CAST IRON

GRAY CAST RON

WHITE CAST IRON

NODULAR CAST IRON

MALLEABLE CAST

IRON

Gray cast iron Gray cast iron, named because its fracture has a gray

appearance Produce by slow cooling. Structure : Graphite in the form of flakes. Properties :

> Advantages : Self-lubricate.

> Disadvantages : Negligible ductility, weak in tension.

Usage : Gear box, head stock, bearing bracket.

Figure 1. Graphite Flakes in Gray Cast iron

Figure 2. Photomicrograph of Gray Cast iron

White cast iron Is called white cast iron because of the white

crystalline appearance of the fracture surface. Produce by rapid cooling. Structure : Iron carbide Properties :

> Advantages : Very hard (difficult to machine), abrasion resistance.

> Disadvantages : Brittle.o Usage : Extrusion dies, ball mills.

Figure 1. Photomicrograph of White Cast Iron

Malleable cast iron Is called malleable cast iron because of latin words

‘malleus’ meaning ‘can be hammered’. Produce by annealing white cast iron at 900˚C for

50hrs. Structure : Graphite exists as clusters or rossetes. Properties :

> Advantages : High ductility, strength and shock resistance.

> Disadvantages : NAo Usage : Transmission gears, connecting rods.

Figure 1. Malleable Cast Iron

Nodular cast iron

Is called nodular cast iron because of graphite is in a nodular or spheroid form.

Produce when gray cast iron with small amounts of magnesium and cerium which nodulates the graphite.

Structure : Graphite in a nodular form. Properties :

> Advantages : High strength and high ductility.

> Disadvantages : NA Usage : Piston, crankshaft.